Rolling bearings have been widely used for many rotating systems including machine tool, turbine, pump, etc. due to their modest cost, low friction, high rigidity and high ability to support combinations of radial and thrust load.
An axial force is required on rolling bearing to reduce clearance between rolling elements and races and to increase stiffness. The axial force is also called preload. However, a large preload may cause a heavy load on the bearing, and adversely affect the fatigue life, temperature rise and power consumption. On the contrary, low preload may lead to larger vibration, excessive sliding between rolling elements and raceways and running error of the spindle. Therefore, a suitable preload is instrumental in reducing friction, heat generation, power consumption and running error of the spindle.
Normally, fixed-displacement method or fixed-load method are used to apply preload to bearings. For fixed-displacement preload, a relative displacement between the outer and the inner races is created in the axial direction. This method is not suitable for high-speed rolling bearings because thermal expansion will induce large load and temperature rise on bearings and result in bearing seizure. A fixed-load preload can be applied by making use of springs to exert an axial force on bearings. Since springs can absorb bearing displacement due to temperature rise, the load exerted on the bearings in the axial direction does not increase with the temperature. Hence, the fixed-load preload is adapted to be used in the high-speed spindle.
Modem high-performance spindles generally operate at wide speed range for higher productivity and quality. Therefore, an adjustable preloading mechanism which enables suitable preload is required in order to satisfy different rotating speeds. Certain preloading regulating structures provided with hydrostatic bearings are disclosed in Taiwanese Patent No. 112785 and U.S. Pat. No. 4,551,032. In this embodiment, the structures are complicated and need many tubes for oil delivery. In addition, certain preloading regulating strictures provided with shape memory alloy are disclosed in Taiwanese Patent No. 173365 and U.S. Pat. No. 5,094,551. In this embodiment, making use of deformation of the shape memory alloy with respect to different temperature, a preload exerted on the rolling bearings can be adjusted. However, the memory alloy is very expensive and still exists many problems in application. It is desirable to develop a preloading regulating mechanism to solve the related problems encountered with the prior arts.